In considering the "dramatic significance" of the play, we might think of  whether Macbeth is a tragic hero.  Act 1 at first tells us he is heroic; we quickly learns he has a fatal flaw,ambition. But is he responsible for his fall & does he learn from his mistake (part of Aristotle's definition of a tragedy). We turn to the witches: do they dictate his fate? Many argue that Macbeth would not have gotten into the mess he did if the witches had not appeared to him in the first place.  Remember, they are never up to any good, but since Banquo resists temptation, certainly Macbeth could have as well. Second, does he learn from his mistake?  In the final scene of the play, he first speaks to MacDuff with hubris, but right before he dies, Macbeth again speaks like the warrior he was at the beginning of the play, telling MacDuff:  “I will not yield, / . . . Yet I will try the last." Although he certainly ends the play bravely, but he doesn't admit he has done wrong or that he is guilty of excessive ambition. To the extent that he does not, he fails as a tragic hero, and that might be the “dramatic significance” of the play.

I would say that to put it very reductively, the "dramatic significance" is "be sure your sins will find you out."  Macbeth and Lady Macbeth are consumed with fear and guilt, both propelled by their insatiable desire for power.

As for the witches, they are yet another warning for Macbeth that his sins will not go undiscovered or unpunished.  They prophesy his fate.  He does not want to believe them, but they speak a bloody truth.  The first witch tells him "to beware Macduff."  The second promises that "none of woman born shall harm Macbeth."  The third tells him that he will be safe until "Birnam Wood moves against him."

The witches prophecies add to the drama because as an audience, we know these things will come to pass, even if Macbeth and his Lady choose to stick their heads in the sand.  It is just a matter of time until each of the prophecies come to pass.